1. Field of the Invention
The present invention relates to an internal combustion engine and, more particularly, to improvements of an internal combustion engine in thermal efficiency.
2. Description of the Related Art
Recently, there is the increasing tendency that internal combustion engines are provided with a fuel injection valve as a fuel feed means and that the fuel injection valve is controlled by a control unit of electronic type, for example, as disclosed in Japanese Patent Publication (kokoku) No. 2-36,772. The internal combustion engine of this kind offers the advantage that an air-fuel ratio of a mixed fuel can be controlled with high freedom. The aforesaid publication discloses an internal combustion engine which is adapted so as to be operated in an air-fuel ratio leaner than the stoichiometric air-fuel ratio, that is, a so-called lean-burn engine.
It is to be noted, however, that the internal combustion engine can provide higher heat frequency in an air-fuel ratio of a mixed fuel leaner than the stoichiometric air-fuel ratio.
The internal combustion engine is so arranged that mechanical work can be conducted by taking advantage of a rise in pressure of a cylinder, caused by the combustion of a fuel fed to a combustion chamber of the engine. Hence, a higher rise in the pressure within the cylinder can provide more efficient work. Given the isometric combustion, a ratio in the rise of the pressure of the cylinder can be represented by the following equation (1): ##EQU1##
where P is the ratio in the rise of pressure in the cylinder;
.epsilon. is the compression ratio;
R is a gas coefficient;
Q is the heat capacity;
V is the volume of the combustion chamber; and
C.sub.v is the isometric specific heat or the constant volume specific heat.
The variation of the temperature relative to the ratio in the rise of the pressure in the cylinder can be given by differentiating the equation (1) above and it can be represented by the equation (2) as follows: ##EQU2##
It is known that the isometric specific heat C.sub.v is made larger as the temperature becomes higher. Hence, the expression, dCv/dT, of the right side of the equation (2) is smaller than zero (0), then the right side of the equation (2) gives a negative value so that the left-hand expression, d(.DELTA.T)/dT, of the left side becomes negative, too.
In other words, that the left side, d(.DELTA.T)/dT, is a negative value means that the higher temperature in the cylinder, T, makes a rise ratio of the pressure in the cylinder, .DELTA.P, smaller, thereby leading to higher efficient work.
On the other hand, it can be noted that there is the relationship between the air-fuel ratio of a mixed fuel and the ratio of the rise in the temperature associated with the combustion of the fuel, i.e. the temperature in the cylinder, T, such that, when the air-fuel ratio becomes leaner than the stoichiometric air-fuel fuel ratio, i.e. when the internal combustion engine is a so-called lean-burn engine, a portion of the heat capacity generated by combustion is absorbed in a surplus amount of the air, thereby leading to lowering the temperature T in the cylinder. Of course, the greater the extent to which the air-fuel ratio becomes leaner, the more the surplus amount of the air, thereby lowering the temperature T within the cylinder.
As is apparent from the foregoing, the lean-burn engine--when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio--lowers the temperature T within the cylinder, thereby making the rise ratio .DELTA.P of the pressure within the cylinder larger, thereby conducing better work. Further, as the air-fuel ratio becomes leaner, the better work can be done and the heat efficiency is further improved.
Actually, the temperature T within the cylinder is so lowered that the transmission of heat to the wall surface of the combustion chamber is reduced, thereby reducing the loss in cooling. Hence, the heat efficiency can be improved from the point of view of the loss in cooling by making the air-fuel ratio leaner.
For the lean-burn engine, the air-fuel ratio has heretofore been restricted in such a manner that the air-fuel ratio of a mixed fuel reaches a predetermined lean air-fuel ratio such as, for example A/F=18, in a low load region and a middle load region (these regions being referred to hereinafter as generally "low load region", in contrast to the high load region as will be described hereinafter). In the high load region, it is common to ensure output in accordance with the load by making the mixture ratio of the fuel gradually larger, that is, by making the air-fuel ratio gradually richer. The same thing can be applied to an engine with a supercharger.
The reasons for making the air-fuel ratio for the lean-burn engine richer in accordance with the load in the high load region are: to gain the output in accordance with the load only by increasing the amount of the fuel because no sufficient extent of the output can be provided solely in the lean-burn state of the engine due to the limit of the ability of the intake system; and to lower the temperature within the cylinder by making the air-fuel ratio richer.
It is to be noted, however, that making the air-fuel ratio rich in the high load region does lead to reducing the heat efficiency in this region. If the heat efficiency would be lowered in the high load region regardless of the fact that the heat efficiency has been made higher by making the air-fuel ratio leaner in the low load region, improvements in the overall heat efficiency of the engine is caused be lowered.
On the other hand, Japanese Patent Laid-open (kokai) Publication Nos. 54-116,512 and 54-980,408 disclose an engine which has an auxiliary chamber having an opening through which it is communicated with a combustion chamber, in addition to a combustion chamber, and an auxiliary chamber valve for opening and closing the opening through the auxiliary chamber. This engine is arranged to generate a strong swirl within the combustion chamber by opening the auxiliary chamber valve at the timing of closing an intake valve and taking advantage of the mixed fuel in the auxiliary chamber, i.e. the pressure differential between the auxiliary chamber and the combustion chamber, particularly in the low load region. It is to be noted, however, that this engine is not arranged so as to reduce the temperature within the cylinder.